As the outcome of the US-Iranian conflict is increasingly being decided at sea, the legal regime of the Strait of Hormuz comes into focus. Pavel Gudev, leading researcher at IMEMO RAS, explores the legal dynamics of transit through the strait, assessing the crisis in the strategic maritime chokepoint through the framework of international law, from ICJ rulings to the UNCLOS treaty.
Peacetime
Legal Characterisation of the Strait
The Strait of Hormuz falls squarely within the category of straits used for international navigation and is governed by Part III of the United Nations Convention on the Law of the Sea. Accordingly, the regime of transit passage applies in full. This is due to two principal factors. First, the narrowest part of the strait between Iran and Oman is approximately 20 nautical miles wide, meaning that the entire water area is covered by the territorial seas of the bordering states. Second, the Strait of Hormuz connects one part of the exclusive economic zone to another (in the Gulf of Oman and the Persian Gulf), thereby satisfying the so-called geographical criterion that triggers the application of the regime of transit passage.
Transit passage is an exceptionally liberal norm of international maritime law: it may not be suspended, it must not be impeded, and it is available equally to all merchant vessels and warships. Submarines may transit in their normal submerged mode, and full freedom of overflight applies, including for military aircraft.
Iran’s Position on the Strait
Iran, as a signatory but not a ratifying state of the 1982 Convention, maintains that the applicable regime in the strait is not transit passage, but innocent passage—a more restrictive rule of international maritime law. Innocent passage must not prejudice the peace, good order, or security of the coastal state; activities such as fishing, marine scientific research, hydrographic surveying, the launching or landing of aircraft, and intelligence-gathering are prohibited.
Iran argues that the regime of transit passage was an innovation introduced by the 1982 Convention and remains a purely treaty-based norm that has not yet crystallised into customary international law (a so-called international custom). Consequently, in Tehran’s view, this regime may be invoked only by states that are full parties to the Convention. As is well known, the United States has neither ratified nor signed it. Iran therefore considers itself under no conventional obligation vis-à-vis the United States to recognise a right of transit passage in the Strait of Hormuz.
Legal Constraints on Iran
At the same time, Iran (as well as Oman), under its domestic legislation, requires prior authorisation for the exercise of innocent passage by foreign warships through its territorial sea. However, it is essential to take into account that Iran is a signatory to the 1958 Convention on the Territorial Sea and the Contiguous Zone, which provides that: “There shall be no suspension of the innocent passage of foreign ships through straits which are used for international navigation between one part of the high seas and another part of the high seas or the territorial sea of a foreign State.”
This implies that Iran cannot insist on an authorisation-based regime for innocent passage, nor may it otherwise obstruct such passage in those parts of the strait overlapped by the territorial seas of Iran and Oman. Regardless of whether other states accept Tehran’s claims, the minimum applicable regime is that of non-suspendable innocent passage; at most, the regime of transit passage applies for full parties to the 1982 Convention.
It should also be noted that any de facto closure of the entire strait by force—including through mining—could amount to a blockade of the ports of Kuwait, Qatar, Bahrain, the United Arab Emirates, and Iraq. This would automatically constitute an act of aggression against those states, as defined in the United Nations General Assembly Resolution 3314 (14 December 1974), which specifies that the blockade of the ports or coasts of a state by the armed forces of another state qualifies as an act of aggression, irrespective of a declaration of war.
Legal Options Available to Iran
At the same time, a series of unlawful acts committed against Iran in the preceding period—including the killing of nuclear scientists—may be invoked as a legitimate basis for countermeasures against suspected states (the United States and Israel being potential targets?). Such measures might include, for instance, the seizure of vessels belonging to those states, rather than the complete closure of the Strait of Hormuz.
These actions could fall within the framework of the doctrine of countermeasures as elaborated by the UN International Law Commission. Under this doctrine, countermeasures are actions that would otherwise be unlawful but are rendered lawful as a response to a prior internationally wrongful act by another state.
Importantly, the detention or arrest of vessels by Iranian authorities would not, in itself, qualify as an armed attack. The judgment of the UN’s International Court of Justice in the 2003 Oil Platforms case (Iran v. United States) indicates that attacks on commercial vessels rise to the level of armed attack only when they are sufficiently grave to threaten the security interests of a state. This interpretation is consistent with UNGA Resolution 3314, which defines aggression as, inter alia, “an attack by the armed forces of a State on the land, sea or air forces, or marine and air fleets of another State”. Prevailing interpretation suggests that the invocation of Article 51 of the UN Charter (individual or collective self-defence) requires an attack on the fleet as a whole or on a substantial part thereof.
The US Blockade as an Act of Aggression
Any blockade in peacetime may be characterised as an act of aggression under the same UN General Assembly resolution, which explicitly prohibits the blockade of a state’s ports or coasts.
This position is directly supported by the 1986 judgment of the International Court of Justice in the case of Nicaragua v. United States. The Court held that “by laying mines in the internal or territorial waters of the Republic of Nicaragua during the first months of 1984, the United States of America has acted, against the Republic of Nicaragua, in breach of its obligations under customary international law not to use force against another State, not to intervene in its affairs, not to violate its sovereignty and not to interrupt peaceful maritime commerce”.
The Court further emphasised that “Turning to the principle of respect for territorial sovereignty, the Court recalls that the principle of sovereignty, both in conventional and in customary international law, extends to the internal waters and territorial sea of every State and to the airspace above its territory. It notes that the laying of mines necessarily affects the sovereignty of the coastal State, and that if the right of access to ports is hindered by the laying of mines by another State, what is infringed is the freedom of communications and of maritime commerce.”
Wartime
Legal Regime of the Strait
The legal framework established by the 1982 Convention for various maritime zones continues, to a significant extent, to apply during an international armed conflict. States are obliged to ensure safe passage through straits used for international navigation for neutral vessels, both maritime and aerial.
Following the logic of the International Court of Justice’s 1949 judgment in the Corfu Channel case, neither a strait state nor any other party may temporarily close an international strait—for example, by laying minefields—to neutral shipping.
The San Remo Manual on International Law Applicable to Armed Conflicts at Sea, adopted in 1994, states that: “The rights of transit passage and archipelagic sea lanes passage applicable to international straits and archipelagic waters in peacetime continue to apply in times of armed conflict. The laws and regulations of States bordering straits and archipelagic States relating to transit passage and archipelagic sea lanes passage adopted in accordance with general international law remain applicable.” (Art. 27). “Belligerent and neutral surface ships, submarines and aircraft have the rights of transit passage and archipelagic sea lanes passage through, under, and over all straits and archipelagic waters to which these rights generally apply.” (Art. 28).
Furthermore, neutral vessels may likewise exercise the right of innocent passage through belligerent international straits and archipelagic waters (Art. 32), and the right of non-suspendable innocent passage ascribed to certain international straits by international law may not be suspended in time of armed conflict (Art. 33).
Iran may also consider all tankers transporting oil and petroleum products in the interests of the United States, Israel, and their allies (and this category could potentially include all Gulf states hosting United States military bases on their territory) as lawful military targets! From the American perspective, “enemy export of certain goods, like crude oil, renders an oil facility or a vessel carrying such commodities a military objective because the enemy earns revenue from the sale of goods that contributes to its war-fighting effort”.
However, in contrast to the American position, prevailing international legal doctrine takes a different view according to which “the majority of States agree that exports from enemy territory transported by neutral vessels or aircraft may legitimately be interfered with only by means of a blockade established and maintained in accordance with the aforementioned rules and principles.”
The United States itself acknowledges (The Newport Manual on the Law of Naval Warfare) that “States are divided on whether civilian objects that contribute to the enemy's war-sustaining effort—as distinguished from its war-fighting effort—are legitimate military objectives. This approach is reflected in some State manuals. In this view, enemy export of certain goods, like crude oil, renders a vessel a military objective because the enemy earns revenue that contributes to its war-fighting effort”. “Another example is the neutralization of enemy shipping carrying strategic commodities, which could cause a strategic effect, thereby curtailing the opposing belligerent’s ability to sustain the war. A majority of States do not recognize that a war-sustaining contribution to the war effort is within the definition of military objectives. According to the majority view, export products carried by neutral vessels cannot be considered military objectives and may be interfered with only by establishing a lawful blockade. Enemy merchant vessels and their cargo, however, are always subject to capture outside neutral waters for supporting the warfighting (or war-sustaining) effort, regardless of whether a blockade has been established”.
The Oslo Manual on Select Topics of the Law of Armed Conflict states that “Fuel, predominantly based on oil, is vital for military operations. Facilities for producing, importing, refining, storing and transporting oil for use by the enemy or its co-belligerents are therefore military objectives by ‘nature’.” “If oil is produced and transported for export, generating revenues to finance the war effort, the facilities fall in the category of war-sustaining industries.” However, the Oslo Manual immediately emphasises that “The lawfulness of attacking such industries, as well as transportation to facilitate export, is contested.”
Iran thus faces a doctrinal choice: whether to treat vessels transporting energy resources as contributing merely to the sustainment of war, or as directly supporting combat operations. European approaches generally recognise only the latter as lawful military objectives, whereas the United States tends to include both categories.
Finally, significant risks for the United States itself accompany statements suggesting that unlawful Iranian interference with navigation (distinct from a formally declared blockade) could lead to the convoying of vessels by US naval forces. Under the San Remo Manual (Article 60), sailing under the convoy of enemy warships or military aircraft may render such vessels lawful military objectives for Iran.
The US Blockade of the Strait
A blockade is a lawful method of warfare in an international armed conflict, but it is subject to strict legal requirements as set out in the San Remo Manual.
First, a blockade must be declared and notified to all belligerent and neutral states, specifying its commencement, duration, location, scope, and the period allowed for neutral vessels to leave the affected area. Second, it must be effective—this is a question of fact, intended to prevent so-called “paper blockades” that exist purely on paper and in declarations alone. Third, vessels breaching the blockade may be captured or attacked under prize law. Fourth, the blockade must be applied impartially to vessels of all states and must not bar access to the ports and coasts of neutral states.
For the United States, this would entail stopping, searching, and inspecting all vessels transiting the Strait of Hormuz—allowing passage to those not acting in Iran’s interests, and detaining those bound to or from Iranian ports. Whether US naval capabilities are sufficient to sustain such an operation remains an open question.
